Plants obtain nitrogen from their environment in the form of inorganic compounds, namely nitrate and ammonia taken up from roots, and atmospheric N2 reduced to ammonia in nitrogen-fixing root nodules. The first step in the assimilation of inorganic nitrogen into organic form predominately involves the incorporation of ammonia with glutamate to form glutamine, catalyzed by the enzyme glutamine synthetase.
Several herbicides function by inhibiting plant glutamine synthetase. A typical example of such compound is the glutamic acid analogue, glufosinate (or phosphinothricin). Many of these herbicides inhibit glutamine synthetase present in the crop plants as well as in weeds, thereby limiting the use of such compounds as glufosinate. Since herbicidal selectivity is important in any commercially useful herbicide, it would be of great interest to be able to confer resistance in selected plants to such non-selective herbicides as glufosinate, as well as to other herbicidal glutamine synthetase inhibitors.
Enzymes that are resistant to herbicidal glutamine synthetase inhibitors are known in the art. Methione sulfoximine (MSO), a glutamate analog, is a mixed competitive inhibitor of pea leaf glutamine synthetase (Leason et al. (1982) Phytochemistry 21:855). Phosphinothricin-resistant alfalfa cells have been reported (Newmark (1983) Nature 305:383-384). The resistance was due to amplification of the glutamine synthetase gene (Donn et al. (1984) Journal of Molecular and Applied Genetics 2: 621-635). The Bar gene, isolated from Streptomyces hygroscopicus, codes for the enzyme phosphinothricin N-acetyltransferase (PAT). This gene can confer resistance to glufosinate herbicides in that PAT detoxifies phosphinothricin by acetylation, which produces an inactive compound.
Additional genes that are resistant to herbicidal glutamine synthetase inhibitors are needed where the resistance is due to a functional mutation in the glutamine synthetase enzyme, rather than an amplification or inactivation by acetylation of the enzyme.